The present invention relates to a control device for loading and unloading mechanism, and more particularly to a lifting height control device incorporated in a fork lift truck. Specifically, the present invention is concerned with a lifting height control device which effects a lifting height control in accordance with lifting height data stored in a microcomputer.
As is well known, a fork lift truck comprises a loading and unloading mechanism and a vehicle body. The loading and unloading mechanism comprises a vertically elongated guide rail hereinafter called an "upright", and a fork slidable in the upright. The mechanism further comprises a hydraulic member, as for example, hydraulic cylinder for lifting and lowering the fork and tilting the upright.
In connection with prior art loading and unloading control devices providing, for instance, lifting height control, the following drawbacks are pointed out. Recently, there has developed a tendency to provide a lifting height which is high when loading and unloading work is effected with a fork lift truck. For instance, the piling and unloading may be required to take place at heights in excess of 10 m. In such a case, it is difficult for an operator to adjust the loading and unloading mechanism so that the fork is placed at the required predetermined height, since the operator is required to look at the top of the fork positioned approximately 10 m above the seat of the operator. Accordingly, it is desirable for the operator to easily effect piling and unloading the load at the predetermined position.
In order to embody this requirement in the prior art, the upright is provided with a limit switch for stopping the fork at a predetermined position. When the fork reaches a predetermined position, for instance, 8.5 m, the control device is designed so as to light a lamp provided at the operator's unit or to break a driving power supply for loading and unloading work. Usually, a load is unloaded on a shelf with a plurality of steps. For this reason, in order to determine the desired position it is required to select the particular step. The provision of a predetermined number of limit switches, for instance ten, is required in order to meet the height of the shelf. Further, the piling and unloading may also be required at another shelf in the working place. In such a case, if the height of the shelf is different from that of the prior one, a more complicated control device is required. Actually, it has been impossible to effect the piling and unloading work. Further, from the point of view of system control in the prior art, a plurality of analog control circuits, such as combinations of relay circuits which are respectively provided for the controlled system, as for example lifting height control, are incorporated in the control unit of the control device for the loading and unloading mechanism. Prior to performing the lifting work, an operator effects various settings according to the lifting height condition required for loading and unloading work and then starts a lifting operation. In this instance, an automatic control system is constituted, which includes therein a valve opening control system provided with respect to a hydraulic pressure circuit for actuating a lift cylinder. The lifting height control is effected so as to control the valve opening control system due to the deviation between an actual lifting height and the above said setting value. However, when the setting is changed to a great extent in accordance with a change in the workpiece for loading and unloading, it is required to adjust the automatic control system in order to stabilize the control system. Alternately, it may happen that the desired control accuracy cannot be obtained. Further, such a lifting height control is effected in a series of control sequences for loading and unloading work with the lifting height control being related to various kinds of controls. Accordingly, it is desirable to supervise the whole system control in view of the simplicity of the circuit and harmonious execution of the control.
In view of this, another attempt has been made. The programmed series of control sequences matching with a target loading and unloading operation is stored in a computer, such as a microcomputer. When, for instance, lifting height control is effected, the appropriate programmed routine for lifting height control is called from the program to effect a lifting height control due to the execution of the programmed routine. In this instance, prior to performing the lifting height operation, the setting is effected by memorizing the target lifting height into the microcomputer. When a push-button for starting an automatic lifting height is pushed, execution of the program for lifting height control routine starts. Thus, the automatic control system including therein the abovementioned valve opening control system becomes operative on the basis of the command being fed from the microcomputer so that the fork moves to the target lifting height to automatically stop thereat. Accordingly, when a change of setting is required, the changed lifting height is memorized, or stored, into the microcomputer. When calling the routines for lifting height control, it is sufficient to call the concerned appropriate routine in such a manner to distinguish it from the other.
In such a computer controlled lifting height control device, prior to storing the target lifting height of the fork, the operation of moving the fork to the objective position is carried out manually. This work has been effected by directly actuating a lift valve with a manual lever, or by controlling a servomotor for controlling the lift valve with a pair of lifting and lowering lever switches. Particularly, when the servomotor is controlled with the lifting and lowering lever switches, an erroneous actuation of lever switches in a loaded condition is dangerous. Further, it is difficult to precisely move the fork to the target position with lever switches. For this reason, in addition to the actuation of lever switches, the actuation of the abovementioned manual lever is required, with the result that the device becomes complicated. Further, when a lifting height is stored in the microcomputer, if the lifting height data are sampled ranging the upper limit of movement of the fork from the lower limit thereof, there occur inconveniences when an automatic lifting height control is effected, due to the unloaded or loaded conditions, or the thickness of the fork. The method of solving such a problem has not been proposed in the prior art. Further, when the lifting speed of the fork is controlled by an automatic lifting height control effected due to the stored lifting height data, if the command for changing the speed is given, it has heretofore been difficult to effect a follow-up control because of the fact that the characteristic of the opening angle of the lift valve with respect to the lifting or lowering speed of the fork is non-linear, and that there exists a response delay inherent in the automatic control system. Furthermore, when the fork reaches the target lifting height and then is stopped thereat, there is not provided a mechanism for slowly stopping the fork. Accordingly, the fork may be stopped suddenly, which may result in a safety problem.